The present invention generally relates to a method for manufacturing a MIS structure on silicon-carbide.
The basic element of a MOS field effect transistor (MOSFET) is the MOS structure composed of what is referred to as a gate electrode, an insulator and a semiconductor. Its electrical properties, which basically define the function and stability of the MOSFET, can be determined, for example, at correspondingly constructed MOS test structures by investigating the capacitance/voltage and current/voltage behavior. The density of locally fixed charges and the density of reloadable traps at the insulator-to-semiconductor boundary surface as well as the strength of the leakage currents flowing through the overall structure are determinant for the quality of the MOS structure. While stationary charges shift the cutoff voltage of a unipolar transistor, traps reduce the mobility of the charge carriers in the channel of the MOSFET. The leakage currents flowing through the MOS capacitor should optimally be small to prevent a charging of traps in the gate oxide and the accompanying degradation of the long-term stability as well as of the transistor characteristics.
The gate electrode of the MOSFET components is usually not fabricated of a metal but of highly doped polysilicon. This, in particular, may be technologically advantageous since self-aligning processes can be utilized in the manufacture of the MOSFETs, and high-temperature processes can be applied after the deposition of the gate material. On the other hand, polysilicon improves those electrical characteristics of the MOS structure that are dependent on the properties of the boundary surface formed by the gate electrode and the gate insulator. No significant degradation of the long-term stability thus arises even when the MOS structure is loaded with strong electrical fields. This is particularly significant for the use of the MOSFETs in the field of power and high-temperature electronics.
The n-channel MOSFET is generally known from J. W. Palmour et al., "High-temperature depletion-mode metal-oxide semiconductor field-effect transistors in beta-SiC thin films," Appl. Phys. Lett. 51, American Institute of Physics, Dec. 14, 1987, pp. 2028-2039, and J. W. Palmour et al., "Semiconductor field-effect transistors in .beta.-SiC thin films," Appl. Phys. Lett. 54, American Institute of Physics, Aug. 15, 1988, pp. 2168-2177, wherein an annularly fashioned gate electrode of polycrystalline silicon that is approximately 500 nm thick and is separated from the SiC semiconductor by an SiO.sub.2 insulator layer is taught. The methods deriving from and optimized in Si technology are employed in its manufacture. Thus, the gate material is deposited from the vapor phase with a chemical vapor deposition (CVD) process at a temperature of T&gt;620.degree. C. and is subsequently doped by drive-in of phosphorous in a very high concentration (&gt;10.sup.21 cm.sup.-3). The MOSFET, however, already conducts current given a gate voltage U.sub.G =0 V (see J. W. Palmour et al. "High-temperature depletion-mode metal-oxide semiconductor field-effect transistors in beta-SiC thin films," Appl. Phys. Lett. 51, American Institute of Physics, Dec. 14, 1987, pp. 2028-2039, FIG. 2, p. 2029, wherein this behavior which is referred to as "normally on" cannot be attributed to the differently sized work functions of gate material and semiconductor.
The 6H--SiC MOSFET disclosed by J. N. Pan et al. in "Self-aligned 6H--SiC MOSFETs with improved current drive," Electronics Letters, Vol. 21, No. 14, July 1995, pp. 1200-1201, also does not switch into the inhibiting condition until the gate electrode lies at a negative potential. Like the above-recited transistor, it can, therefore, not be utilized in all areas of circuitry and power electronics where self-inhibiting ("normally off") MOS components are required.
As taught in W. Xie et al., "The effect of thermal processing on polycrystalline silicon/SiO.sub.2 /6H--SiC metal-oxide-semiconductor devices," Appl. Phys. Lett. 68, American Institute of Physics, Apr. 15, 1996, pp. 2231-2233, the influence of the process temperature on the creation of stationary charges was investigated during the manufacture of a poly-Si/SiO.sub.2 /6H--SiC component. As taught in this publication, the dopants are recommended to be driven into the gate material at a temperature T.about.850-900.degree. C. to limit the effective density of stationary charges to values Q.sub.tot &lt;8-9.multidot.10.sup.11 cm.sup.-2.